1. Field of the Invention
The present invention relates to systems and methods of water purification, and, more particularly, the control of nutrients and suspended and filamentous algae in estuaries and fresh water bodies.
2. Description of Related Art
Many freshwater lakes and ponds, as well as estuaries, are characterized, particularly during the warmer months, by a significant population of suspended algae or phytoplankton in the water body's water column. These largely unicellular plants give the water a greenish and often a “pea-soup” appearance that many observers find unattractive. Floating mats of unsightly, filamentous algae also can occur. High concentrations of algae may lead to low levels of dissolved oxygen in the early morning hours, leading to stress on the aquatic and fish populations. In extreme cases, these conditions will lead to fish kills and the general decline of the quality of a water body.
The basis of the problem is an abundance of soluble nutrients within the water body, which then allows the rapid growth and maintenance of the elevated population of suspended or filamentous algae. The source of the soluble nutrients may be sediments, air deposition, point source polluting discharges, generalized, non-point-source inflows, or most likely a combination of all these factors. An effective management strategy would combine elements of attempting to reduce nutrient loading to the water body with treatment of the water body itself.
Currently used methods of controlling algal growth in ponds or lakes typically involve treating the water with selected herbicides or “algicides.” These chemicals kill the suspended algae, returning the water to its more desired appearance of clear or only slightly colored waters. Alternatively, a water body may be treated with various aluminum salts (e.g., aluminum sulfate), which achieves a similar result through a chemical precipitation reaction. Another strategy employed in managing algae is to introduce a dye that then, via the mechanism of shading, achieves the same result of killing the algae and returning the water to its algae-free appearance.
Another problem with these approaches is that the underlying feature that initially encouraged the growth of the algae remains; that is, the nutrients on which the algae grew remain in the water, and after the effects of the algicide, herbicide, or dye decrease, the conditions for a renewed growth of algae are abundantly present. In addition, for the algicide and herbicide at least, the negative environmental effects of potentially toxic accumulation must be considered. In the case of aluminum salts, a temporary reduction in selected nutrients (e.g., phosphorus) is effected, but considerable skill and expertise are required to effectively and efficiently precipitate the suspended (nutrient-containing) solids.
An alternative strategy to killing the algae and then creating relatively clear but nutrient-rich water body is to cause a release of nutrients from the algae but then to remove these nutrients from the water body. A natural method of achieving this nutrient removal is through the harvesting of macrophyte vegetation, which takes up the soluble nutrients as a function of their growth. If the total mass of nutrients removed through plant harvest were to match the ongoing nutrient loading through the various sources of sediment transport, air deposition point and nonpoint sources, then the lake or pond would be able to maintain an algal-free appearance.
In U.S. Pat. No. 4,888,912, a system of growing and harvesting aquatic plants is described. In it, an “envelope,” made of a plastic material permeable to light, air, and water, contains certain plants (typically submerged macrophytes), which then grow and assimilate soluble nutrients. Because the plants are contained in the envelope, harvesting is convenient and the plants are prevented from escaping into the larger water body.
The ability of the plants within the envelope to grow, and hence reduce the population of algae depends on a complex ecological competition between the introduced species of macrophyte vegetation and the existing algae in the water body. The nutrients contained in the healthy algal biomass are normally bound up in cellular constituents—that is, not biologically available for other plants' growth. The introduced plant species must then effect a decrease in the standing crop of algae, thereby creating an increase in the amount of nutrients bioavailable for the introduced species. Therefore, using introduced macrophytes to decrease ambient soluble nutrient levels and algae populations is necessarily a two-step process. First, the nutrients bound up in the algal biomass must be released and become bioavailable. Second, the soluble nutrients must then be incorporated into a standing crop amenable to periodic harvesting. The harvesting effectively removes the nutrients from the water body.
Shading of the suspended algae-containing water is one means to achieve algal cell lysis and an increase in the proportion of nitrogen and phosphorus in the water column that is biologically available for other plant growth. The use of shading to cause a release of soluble nutrients has been investigated and taken advantage of in the prior art. In U.S. Pat. Nos. 5,096,577, 5,180,501, 5,264,127, 5,342,512, and 5,409,601 (all assigned to the Lemna Corp.), a floating or submerged aquatic plant is used to cover and shade a wastewater beneath.
Reddy and DeBusk (1987) determined in a short-tern mesocosm-scale experiment that the primary nutrient removal mechanism in a water hyacinth system that received phytoplankton-laden lake water was the settling of algal cells.
In U.S. Pat. No. 4,042,367 the introduction of a colored dye is used to prevent the transmission of photosynthetically active radiation through a water column. In turn, this action prevents photosynthesis and thereby controls the growth of algae.
The Florida company Amasek, Inc., working on Lake Apopka and Round Lake in Florida in the late 1980s and early 1990s, grew water hyacinths within the confines of in situ boom and barrier systems. Through the process of water shading, the hyacinths were able to outcompete the suspended algae through algal lysis, nutrient release, and subsequent uptake by the hyacinths.
The sustainable removal of nutrients, however, involves not just shading and subsequent plant uptake and plant harvest. Many macrophytes, such as the floating water hyacinths or submerged macrophytes, are characterized by much higher growth rates than can be adequately sustained by the amount of nutrients held by a water column beneath them. Reddy et al. (1983) concluded that the high levels of floating water hyacinth biomass in a central Florida lake could be sustained only by transport of nutrients from the sediments and detritus or by fixation (in the case of nitrogen) from the atmosphere.
A particular model of a system for decreasing algal concentration consists of macrophyte vegetation (plants) and various permutations of floating boom, barrier, and water control mechanisms (pumps, internal barriers, etc.) for sequentially shading suspended algae-containing water. The shading causes the algae to lyse and release contained nutrients, which then promotes the growth of a standing crop of either subsurface (submerged) or floating vegetation. Depending on the configuration desired, the plants are periodically harvested from the containment system, effectively removing soluble nutrients such as phosphorus and nitrogen from the pond, estuary, or lake. In the case of submerged macrophytes, water chemistry changes caused by their photosynthetic activity can further contribute to nutrient removal (e.g., co-precipitation of phosphorus with calcium carbonate) beyond that achieved by plant harvest. As the overall total level of nutrients within the water column decreases, the conditions favorable to sustained nuisance algal growth diminish, and further algal growth is discouraged.
Therefore, to encourage further plant growth and hence continued removal of suspended and filamentous algae, it is believed desirable that the water beneath the macrophyte vegetation, whether held within a barrier or not, be exchanged with new nutrient-bearing water.